Transport systems such as underground railway, tram, trolleybus and guided road vehicle systems have high power requirements which are unevenly distributed through the day and essentially consist of the traction power that is dissipated by the vehicles of the system. The power requirements peak in the morning and in the afternoon, at times corresponding to the morning and evening rush hours. It is therefore not advantageous for companies operating these transport systems to acquire their own electrical power stations to supply electrical power, since the power stations would have to be rated to cater for the peak power in the rush hours, and would rarely be operated at their maximum efficiency.
It is therefore more economic for companies operating these transport systems to obtain electrical power from external electrical power distribution companies in the public or private sector. These companies operate electrical power supply networks that provide a very reliable supply and are interconnected to deal with peak power demand. However, most power distribution company price structures take account of a nominal power and energy specified in a contract negotiated with the client, and they bill clients for high additional costs whenever the power or the energy consumed by the client exceeds the nominal power or energy. It is therefore important for operators of these transport systems to optimize the peak power drawn from the external electrical power supply network, peak limiting having a significant financial impact by limiting the nominal power itself and the occasions on which power demand exceeds the nominal power. The same applies for the electrical energy consumed.